This invention relates to pixellated devices such as active matrix liquid crystal displays, and particularly to methods of fabricating the substrate with the active matrix circuit, known as the active plate, used in the manufacture of such devices.
An active matrix liquid crystal display (AMLCD) typically comprises an active plate and a passive plate between which liquid crystal material is sandwiched. The active plate comprises an array of transistor switching devices, typically with one transistor associated with each pixel of the display. Each pixel also has a pixel electrode on the active plate to which a signal is applied for controlling the display output of the individual pixel.
FIG. 1 shows the electrical components which make up the pixels of one known example of active plate of an AMLCD. The pixels are arranged in rows and columns. The row conductor 10 of a pixel is connected to the gate of the TFT (thin film transistor) 12, and the column electrode 14 is coupled to the source. The liquid crystal material provided over a pixel electrode of the pixel effectively defines a liquid crystal cell 16 which is connected between the drain of the transistor 12 and a common ground plane 18. An optional pixel storage capacitor 20 is connected between the drain of the transistor 12 and the row conductor 10 associated with an adjacent row of pixels.
For transmissive displays, a large area of the active plate is at least partially transparent, and this is required because this type of display is illuminated by a back light. In these display devices, the pixel electrode must be transparent, whereas row and column conductors are usually formed as metallic lines which are opaque. Metallic layers, such as chromium, aluminium, alloys or multilayer structures are used for the row and column conductors because of the high conductivity, which improves the device performance. The conductivity of the lines (usually the column lines) to which the pixel drive signals are applied is particularly important in large displays, because a sizeable voltage drop occurs over the length of the line, making it impossible to drive uniformly all pixels along the line (column).
A problem with the use of metallic column conductors is that separate deposition and lithographic procedures are required to form the column conductors and the pixel electrodes. The pixel electrodes must be transparent, and are typically formed from a transparent conductive oxide film. It is well known that the lithography steps in the manufacturing process are a major contributing factor to the expense of the manufacturing process. Each lithographic step can be considered to reduce the yield of the process, as well as increasing the cost.
The conventional manufacturing process for the active plate of an LCD is a five mask process. With reference to the bottom gate TFT LCD active plate shown in FIG. 2, the process steps, each requiring a separate mask definition, are:
(i) defining the gate 22 (which is part of the row conductor) over the substrate 21;
(ii) defining the amorphous silicon island (which overlies a gate dielectric 23 that covers the entire structure), comprising a lower intrinsic layer 24 and an upper doped contact layer 26;
(iii) defining the metallic source 28, drain 30 and column electrode 32;
(iv) defining a contact hole 34 in a passivation layer 36 which covers the entire substrate; and
(v) defining the transparent pixel electrode 38 which contacts the drain 10 through the hole 34.
The capacitor shown in FIG. 1 may simply be formed from the gate dielectric by providing an area of overlap of one pixel electrode with a portion of the row/gate conductor of the adjacent row.
There have been various proposals to reduce the number of lithography steps, and thereby the mask count, of the manufacture process in order to reduce cost and increase yield.
For example, it has been proposed to form the column conductors from the same transparent conductive oxide film as the pixel electrode, so that these components of the pixel structure can be deposited and patterned together. Additional measures can result in a two mask process, and this is explained with reference to the bottom gate TFT LCD active plate shown in FIG. 3. The process steps, each requiring a separate mask definition, are:
(i) defining the gate 22 (and row conductors); and
(ii) defining the transparent column electrode 32 (which also forms the TFT source 28) and the pixel electrode 38 (which also forms the TFT drain 30).
The definition of the semiconductor island 24, 26 can be achieved by a self-aligned process using the gate 22, for example by using light exposure through the substrate. Of course, the semiconductor could equally be formed with a third mask step (between steps (i) and (ii) above). In the periphery of the array, the gate dielectric 23 is etched away using a low-precision stage, to allow contact to the gate lines at the periphery of the display.
In this structure, the high resistivity of the transparent conductive oxide film used for the column lines prevents the use of the structure in large (TV-sized) displays.
For this reason, there are further proposals to treat the column conductor area of the layer to increase the conductivity, whilst not affecting the transparency of the pixel electrode. For example, the article xe2x80x9cConductivity Enhancement of Transparent Electrode by Side-Wall Copper Electroplatingxe2x80x9d, J. Liu et al, SID 93 Digest, page 554 discloses a method of enhancing the conductivity by electroplating a copper bus to the sidewall of the metal oxide column line. The process involves an incomplete etching process to leave metal oxide residues, which act as seeds for the copper growth. The process is complicated and difficult to control. In addition, the copper bus will surround the source and drain electrodes, and there is a risk of shorts between the source and drain resulting from fast lateral copper growth when forming the bus. The copper bus around the source and drain electrodes also influences the channel length of the TFT and therefore makes the TFT characteristics less predictable.
WO 99/59024 discloses a method for enhancing the conductivity of a transparent electrode by providing patterned metallic layers adjacent to the transparent electrodes.
There is still a need for a simple process for increasing the conductivity of a transparent metal oxide layer, such as ITO, without increasing dramatically the complexity of the process. Such a process will find for example application in active matrix LCD manufacture but will also be useful for other technologies where mask count reduction could be achieved if a transparent conductive layer could be made more conductive in certain areas without losing the transparency in others. This may be of benefit in, for example, polymer LED displays and large area image sensors.
According to a first aspect of the invention, there is provided a method of fabricating an active plate comprising pixel electrodes and associated address lines formed from a transparent conductive material, which method comprises:
providing a transparent conductive material layer and a metal layer in succession over a substrate,
depositing and patterning a shielding layer into a configuration corresponding to the desired pattern of the transparent conductive layer required for the pixel electrodes and the address lines, the shielding layer being formed in a manner such that an etching property of the shielding layer at regions corresponding to the pixel electrodes differs from that at the regions corresponding to the address lines,
subjecting the shielding layer to an etching process using the difference in properties so as to remove the regions of the shielding layer corresponding to the pixel electrodes while leaving portions of the shielding layer at the regions corresponding to the address lines,
and thereafter removing the portions of the metal layer at the regions corresponding to the pixel electrodes. The remaining portions of the shielding layer may subsequently be removed.
Through this method, an active plate is fabricated in which the pixel electrodes comprise transparent conductive material and the associated address lines comprise transparent conductive material, from the same deposited layer as the pixel electrodes, together with an overlying coating of metal. The address lines are thus of a composite nature, comprising a combination of the transparent conductive material with overlying metal, and consequently, the effective electrical conductivity of the address lines is considerably improved compared with those of the known method comprising transparent conductive material alone. At the same time, the complexity of the kind of plating method entailed in the approach proposed by Liu et al is avoided.
The invention provides a simple process that produces address lines with reduced resistivity while maintaining high transparency of the pixel electrodes. The process is compatible with an overall simple, low mask count, fabrication method suitable for making active plates for AMLCDs and the like and satisfies the required characteristics of line conductivity and pixel transparency for producing devices of large size and/or high resolution. Importantly, the invention also has the advantage of being compatible with existing manufacturing equipment.
The shielding layer preferably comprises photoresist.
In a preferred embodiment, the property of the shielding layer which differs for the pixel electrode and address line corresponding regions and which affects the etching characteristic comprises thickness of the layer, and in this case the thickness of the shielding layer is made greater at the regions corresponding to the address lines than at the regions corresponding to the pixel electrodes.
When using photoresist for the shielding layer, the patterning of the layer into selected regions of different thickness can conveniently be accomplished using a photolithographic patterning technique of the kind described by C. W. Han et al in the paper entitled xe2x80x9cA TFT manufactured by 4 masks process with new photolithographyxe2x80x9d published in SID Proceedings of the 18th International Display Research Conferencexe2x80x9d, (Asia Display ""98), pages 1109-1112. This technique involves the use of a photomask which, in addition to light blocking and transparent areas, has regions with grid or slit patterns. These regions, through diffraction effects, control the extent of exposure of the photoresist and thus the thickness of the resulting photoresist, the thickness being less than that produced by light-blocking regions of the mask.
It is envisaged that other techniques could be employed to produce selected regions of different thickness in the shielding layer and also that the property of the shielding layer which differs, and which affects the etching characteristic of the layer such that certain areas can be selectively etched away while leaving shielding layer material at other areas, could be other than thickness.
According to a second aspect of the invention, there is provided a method of manufacturing an active plate for a liquid crystal display, comprising:
depositing and patterning a gate conductor layer over an insulating substrate;
depositing a gate insulator layer over the patterned gate conductor layer;
depositing a silicon layer over the gate insulator layer;
depositing a transparent conductor layer over the substrate;
depositing a metal layer over the transparent conductor layer;
depositing and patterning an etchable shielding layer over the metal layer, the shielding layer having a configuration defining source and drain areas, pixel electrode areas, and line conductor areas associated with the source or drain conductors, the regions of the shielding layer defining the line conductor areas having a thickness greater than that of the regions defining the pixel electrodes;
patterning the transparent conductor layer and the metal layer using the shielding layer;
partially etching the shielding layer to remove the thinner regions so as to expose the metal layer at the pixel electrode regions;
and removing the metal layer regions at the pixel electrode regions.
This method can enable a two mask process to be used to fabricate the active plate, and in which the conductivity of the address lines comprising transparent conductor material being improved by overlying metal layer regions, in the case where the silicon layer is self aligned to the gate conductor.
The invention is preferably used for making the active plate of an active matrix liquid crystal display. The invention also provides an active matrix liquid crystal display comprising this active plate, a passive plate, and a layer of liquid crystal material sandwiched between the active and passive plates.